Bridges Reach analysis Fundamental tool for design Scalable and can be phased 3 defined levels of analysis

Bridges Step-wise sizing method Existing bridge condition and history Confined channels Floodplain channels Floodplain management Flood control features Infrastructure

Which is best? No Slope short, low slope streams (<75’ , <3%) simple situations (single lane road, low fill) confined to moderately confined channels Stream Simulation default channels <15 ft steeper, longer culverts improved ecological connectivity required moderately confined channels Hydraulic Method retrofits exceptional circumstances Bridge default channels >15 ft wide, steep, or high energy streams high potential for vertical or horizontal instability high debris or sediment load

Survey minimum 40 channel widths Long Profile Long profile is necessary to understand reach-level behavior of the channel identify grade breaks assess regrade potential and effects average scour depth of pools determine channel incision channel characteristics for stream simulation In alluvial channels Survey minimum 40 channel widths plus culvert length Chapter 7, P. 40

Outlet drop caused by scour pool vs. incised channel Outlet Scour Original Channel Grade Incised Channel Original Channel Grade Nick point supported by culvert Incised Channel Grade

4 ft

Channel profile Scale of Problem = Scale of Solution Outlet scour – Solution at culvert scale Short Incised channel – Reach solution; regrade Expected regrade Long

Channel Regrade Options Grade control (or fishway) Maintain upstream bed elevation to protect current habitat or infrastructure Build up as options

Channel Regrade Options Grade control (or fishway) Build up as options Increased reach gradient leads to Greater transport capacity Increased downstream velocity Decrease in habitat quality

Channel Regrade Options Grade control impacts Required to maintain culvert within design criteria (part of culvert design) Locks-in channel section in a vertically mobile channel system (often leads to more controls downstream). Requires maintenance for the life of the culvert (obligation of the landowner). May reduce habitat value of reach. Build up as options

Channel Regrade Options Grade control (or fishway) Min 25 Min. 35- 50 ft Build up as options Downstream bed control to anticipate further channel incision. These recommendations are not in the culvert manual.

Channel Regrade Options Allow upstream bed elevation to lower and establish a new profile. Regrade Build up as options

Channel Regrade Options Build up as options

Channel Regrade Options Regrade impacts Temporary upstream habitat loss Possible permanent loss of habitat (exposure of bedrock) Downstream sediment deposition Potential lowering of local water table, perching riparian Build up as options

Channel Regrade Options Regrade benefits Reestablishment of natural stream profile and processes Rejuvenation of upstream habitat Elimination of man-made structure in stream Build up as options

Channel regrade considerations Extent of regrade expected. Low channel slopes (fine bed materials) are sensitive. High channel slopes less so. Condition of upstream channel and banks Are they susceptible to bank collapse, riparian dewatering? Habitat impacts by upstream incision Is there value of culvert as nick point, preventing further incision? Is there hard pan or bedrock near the surface? Culvert and channel capacity reduced by sediment slug released from upstream incision. Risk to utilities and structures. Potential passage barriers created upstream by further incision. Construction access to dig regrade. Chapter 7, p. 41-43

Incising upstream channel Slug of regraded material plugging the culvert.

Are these “plans” good enough?

Complete plans include: Project Plans Complete plans include: Project layout New and old culvert profiles superimposed Long channel profile Bed material specifications and quantities Care of water (erosion control, diversion plans) Road runoff treatment

Project Plans

Project Plans

Project Plans

Stream Simulation Culvert Effectiveness Study WDFW and DNR Bob Barnard Sheila Yokers Heather Tschaekosfske Alex Nagygyor

Reference Reach Culvert

10% (5) culverts with D50 larger than reference Sites arranged in order of decreasing sediment size →

Thalweg Depth survey

10% (5) culverts with deeper mean thalweg depth than reference Reference reach 95% CI envelope 44% (22) culverts with shallower mean thalweg depth than reference

6% (3) culverts with Q100 velocity greater than reference 11% (5) culverts with Q100 velocity less than reference